This invention relates to a hydraulic system for an internal combustion engine with at least one hydraulically actuated coupling element such as a sliding coupling, preferably in a switchable valve drive element such as a cam follower or support element for it. The valve drive element is preferably of the type having at least two parts that move with respect to one another to attain different cam strokes, wherein on at least one side of the coupling element, a path runs in or on the valve drive element to feed switching hydraulic fluid pressure. The valve drive element moves within a bore in the internal combustion engine into which a first channel opens to feed the switching hydraulic fluid pressure, one end of the channel being supplied by a hydraulic fluid pump followed by a directional valve to turn on and turn off the switching pressure, and the other end of the channel being hydraulically connected to the path.
Hydraulic systems of this type in an internal combustion engine, for example to actuate a coupling element of a switchable valve drive element such as a flat tappet, roller tappet, support element or finger lever or rocker arm or the like, exhibit a list of system-dependent disadvantages (see also DE 196 04 866 or U.S. Pat. No. 5,351,662). When a switch command is issued, delays or fluctuations in the switching time occur that are dependent on RPM, temperature, wear, tolerances or oil viscosity. An important factor influencing the delay in the switching time is the undesirable high compressibility of the hydraulic fluid used caused by entrained air bubbles or oil foaming that occurs on top of the inherent compressibility of the hydraulic fluid that always exists but is relatively minimal. These air bubbles can make their way bit by bit, for example, into a hydraulic fluid feed channel ahead of its respective coupling element when the internal combustion engine is shut off and the channel is idle, even if the corresponding switchable valve, or a check valve, prevents backflow out of the channel. After starting the internal combustion engine, this channel must be bled long enough prior to the first switch command until any amounts of air are removed from it, or at least most of it is. However, there are always areas in this channel that are at geodetically high relative points or at the end of the channel, for example directly in front of the corresponding coupling element, that despite everything are not affected by the bleed stream produced in the channel by the pump-channel connection effected by the switchable valve. A person trained in the art could of course install bypasses around the switchable valve, for example, in order to produce a permanent bleed stream, or to define blow down or leakage points, but this unnecessarily increases the design effort and the costs associated with the hydraulic system and with the overall internal combustion engine.
Also, the hydraulic fluid used can foam up, for example when the internal combustion engine is at hot idle. This foaming can also lead to the undesired hydraulic fluid compressibility mentioned. In worst case, after issuing the switch command, no switching occurs at all at the coupling element since all that occurs is the compression of air or oil foam. Here, as well, a permanent bleed stream could be used to remove the undesired air as much as possible from the corresponding coupling channel when the hydraulic fluid is disconnected. However, as mentioned, this bleed stream does not reach the entire range of the channel up to directly in front of the coupling element, resulting in this air cushion merely being pushed back and forth in the channel between coupling cycles.
The object of this invention is thus to create a hydraulic system of the above type in which the disadvantages cited are remedied using simple means.
According to the invention, this object is met by providing the path with a connection to a second hydraulic fluid channel at least near the coupling element. This second channel feeds high-pressure hydraulic fluid when the switching pressure is shut off in the first channel, the pressure in the second channel being less than the necessary switching pressure.
Useful embodiments of the invention are discussed below, which can also include independently protectable features.
At this point, it is stressed that the area of protection of the invention refers in particular to a hydraulic system of an internal combustion engine and here especially to a hydraulic system to actuate a coupling element for a slide coupling of a switchable valve driver. However, the concept of the invention goes so far as to include a multitude of hydraulic systems in the design of engines, as well as in other technologies where a slide valve or similar element is to be hydraulically shifted. For example, the invention can also be used for block pistons or slide valves in hydraulic camshaft positioning devices. Also, the area of protection does not extend only to valve drive elements that are installed in slots or bore holes in internal combustion engines, but for example can also extend to finger levers, valve rockers or rocker arms next to one another that can be coupled together selectively using at least one hydraulically moving slide coupling.
According to the invention, by producing a connection to a second high-pressure channel directly in front of the coupling element or its path in the valve drive element, the first channel can be completely or almost completely bled free of air bubbles during the critical times described above, at least while the coupling pressure in the first channel is disconnected. It is helpful in the process to keep this xe2x80x9cbleed pressurexe2x80x9d to a minimum so as to prevent it from moving the coupling elements in their movement direction. This measure can be implemented extremely inexpensively, for example by means of a simple notch in the valve drive element, as illustrated below in more detail.
Although it is not required, it is expedient to make use of a second channel that supplies a hydraulic play-equalization element in the valve drive element. It is however possible to use separate controls as well.
In this manner, the connection from the second channel to the path directly in front of the side of the coupling element or directly near the side of the coupling element is created by means of a pressure-reducing design such as anozzle or a throttle. This allows the full hydraulic fluid pressure, which is used to actuate the hydraulic play-equalization element to be turned on since only a single hydraulic fluid pump is used, which is an advantage. This is because directly in front of the path or coupling element, the pressure is reduced.
If a person skilled in the art is able to place the connection between the second and the first channel directly behind the side of the coupling element, the best success can be expected according to the invention.
Instead of the suggested nozzle or throttling device, there are other pressure-reducing measures that are already available to the person trained in the art.
According to an additional embodiments of the invention, the hydraulic system can be applied to a hydraulic flat tappet. Here, the xe2x80x9cbleed pressurexe2x80x9d should pass from a supply chamber in the flat tappets fed from the second channel to a supply chamber in the flat tappets fed from the first channel. In this manner, a transfer line can be implemented at an edge region at the bottom of a slot for the coupling means in the flat tappet, it being useful to locate said slot near the base (but not necessarily).
Another preferred embodiment of the invention relates to a hydraulic system of a roller tappet or similar device. Here, the connection can be produced as an axial path that leads from the second channel at the bore hole in the internal combustion engine to the path in front of the coupling element.